"Caster sugar is the name of a very fine sugar in Britain, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. It is sold as "superfine" sugar in the United States." (Definition from Ochef)

I believe it's what C&H sells in the U.S. as "Baker's Sugar," which I used for a while. It comes in a box that resembles a ½-gallon milk carton. It's a bit more expensive than C&H's regular sugar, and a whole lot more expensive than store brands. It's supposed to dissolve better, but I have never had any problems with regular sugar in any of the baking I do. Also, I've found that once opened it tends to clump much more than regular sugar. I decided it wasn't worth the extra cost and went back to regular.

I believe it's what C&H sells in the U.S. as "Baker's Sugar," which I used for a while. It comes in a box that resembles a ½-gallon milk carton. It's a bit more expensive than C&H's regular sugar, and a whole lot more expensive than store brands. It's supposed to dissolve better, but I have never had any problems with regular sugar in any of the baking I do. Also, I've found that once opened it tends to clump much more than regular sugar. I decided it wasn't worth the extra cost and went back to regular.

It is also possible to use brown sugar or white sugar. My mom uses brown sugar and the result is just the same